HYDRO-MECHANICAL PROPERTIES AND STABILITY ANALYSIS OF FOUR LANDSLIDE-PRONE HILLSLOPES IN WESTERN NORTH CAROLINA

Rainfall-induced landslides present a hazard to people and property in western North Carolina. In the last century extreme rainfall events, many associated with tropical cyclones, have triggered widespread landsliding and debris flows and have resulted in at least 48-recorded fatalities in the region. Although, advances in weather prediction and availability of high-resolution topographic information provide valuable inputs for forecasting landslide potential using hydro-mechanical models, these models require knowledge of soil properties and moisture conditions that cannot be remotely sensed. Therefore, we identified four steep, (average 31°) landslide-prone, unchanneled catchments in the Blue Ridge Geologic Province of western North Carolina located near past debris flow activity to monitor the soil-water response to rainfall infiltration. In general, the soil profile at the sites consists of a 0.1 to 0.3-m thick organic horizon that overlies a colluvial horizon that extends to weathered bedrock located 0.8 to 1.8 meters below the ground surface. We present results of geotechnical and hydrological tests that are used as inputs to slope-stability models to assess the potential for landslide initiation at the sites. Soil-water and suction stress characteristic curves were obtained for intact specimens using the Transient Release and Imbibition Method (TRIM). Direct-shear tests under saturated and field conditions were used to obtain Mohr-Coulomb failure parameters. Results of infinite-slope stability analysis indicate that pore-water pressures ranging from 4.6 to 11 kPa are needed to initiate landslide movement at these locations.